1. Field of the Invention
The present invention relates to an image forming device that performs optical recording on an image bearing member using a plurality of light sources, and more particularly to an image forming device wherein an automatic power control (APC) is used to adjust the intensity of the light sources.
2. Description of Related Art
In an image forming device, such as a laser printer, optical recording on a photosensitive drum is usually performed by a laser chip including a semiconductor laser serving as a light source, and a photodiode serving as a detector. The optical recording is performed in such a manner that a polygon mirror scans a laser beam modulated by an image signal onto a photosensitive drum. The photodiode detects a part of the laser beam and a detection signal output from the photodiode is fed back to the semiconductor laser to ensure that the intensity of the output from the semiconductor laser is maintained at constant. This type of control is referred to as an APC. APC control is normally performed each time for one line main scan to improve image reliability. This control will hereinafter be referred to as “line APC control”.
Japanese Patent Application Publication No. 11-101947 proposes a multi-laser beam printer equipped with a plurality of semiconductor lasers. Such a printer complies with requirements that the laser printers be operated at a high speed and images printed thereby be at high precision. Multi-laser beam printers can form multiple image lines in one scan and print several times faster than a normal print speed without raising the rotational speed of the polygon mirror.
The laser printer disclosed in the Japanese Patent Application Publication performs a line APC control for each semiconductor laser and therefore requires one sample-and-hold capacitor for each semiconductor laser. During sampling, the sample-and-hold capacitors adjust the light amount of the semiconductor laser and maintain the adjusted light amount during the hold state.
However, each sample-and-hold capacitor in a conventional multi-laser beam printer requires a switching circuit to switch the capacitors between a sampling state and a holding state. Thus as many switching circuits as there are semiconductor lasers must be provided, which is costly.